This invention relates to aqueous solutions of aminomethylenephosphonic acid (hereinafter referred to as "AMP acid"), selected from diethylenetriaminepenta(methylenephosphonic acid), diethylenetriaminetetra(methylenephosphonic acid) and diethylenetriaminetri(methylenephosphonic acid) (hereinafter designated "D5A", "D4A" and "D3A" respectively), having improved storage stability. The aminomethylenephosphonic acids are useful as sequestrants for metal ions.
Aqueous solutions of AMP acid are existing commercial products, and these usually contain a mixture of D5A, D4A and D3A, of which the principal component, providing at least 40%, e.g. from 55% to 85% and usually from 60% to 80%, of the total weight of D5A, D4A and D3A, is D5A. The remainder is normally mostly D3A together with a small amount of D4A. Other phosphonic acids, e.g. hydroxymethylenephosphonic acid, may be present in trace amounts. Typical commercial products also contain small amounts of phosphorous acid and hydrochloric acid (HCl), the latter in an amount less than 10% by weight, usually not more than 9%, by weight of the solution. (In the present description and claims, all percentages are by weight, i.e., grams per 100 grams of solution.)
Conventionally, the commercial aqueous solutions are described in terms of their concentration of "active phosphonic acid" which is measured by titrating the phosphonic acid groups in such solutions and converting the results, using predetermined constants, to the stoichiometrically equivalent concentration of D5A. The concentration of AMP acid, expressed in this way as active phosphonic acid, is normally from about 90% to about 99% and most commonly from about 94% to about 98% of the actual concentration of AMP acid.
To minimize handling the transportation costs, it is preferred to ship and store the AMP acid in relatively concentrated solutions, which typically contain from 40% to 55% of active phosphonic acid. A problem with such solutions, (although the problem also exists with more dilute solutions), is a tendency for the AMP acid to crystallize, particularly if the solutions are subjected to sudden changes in environment within a storage temperature range which would be normally from about -5.degree. C. to about 40.degree. C., (most usually 5.degree.-35.degree. C.) but which exceptionally may be as low as about -20.degree. C. or as high as about 50.degree. C.
It might be supposed that solubility of the AMP acid would be suppressed by the common ion effect of the hydrochloric acid in the solution, and that a reduction in the concentration of the HCl might provide an improvement. Alternatively it might be supposed that an increase in the concentration of HCl in the solution, or the addition of some other mineral acid to the solution, might aggravate the problem. In fact, this result is obtained in the case of other aminopolymethylenephosphonic acids such as, e.g. nitrilotri(methylenephosphonic acid) as the limit of its solubility in aqueous solutions at ordinary temperatures (e.g. 25.degree. C.) is driven to very low levels by the presence of HCl in concentrations above 10% of such solutions.
Surprisingly, it has been found that solutions of AMP acid do not show the behavior which would be expected from such considerations, and that solutions containing higher concentrations of HCl than the usual solutions referred to above, i.e., solutions containing 10% or more of hydrochloric acid, have improved stabilities. Other mineral acids have been found to have a similar effect.
Accordingly, the present invention provides aqueous solutions of AMP acid having improved stability against crystallization of the AMP acid, and a method for stabilizing aqueous solutions of AMP acid against such crystallization.